1. Field of the Invention
The present invention relates to the use of nonwoven layers in a laminated construction to produce a papermakers fabric.
2. Description of the Prior Art
In papermaking machines, a papermakers fabric or belt in the form of an endless belt-like structure is supported on and advanced by various machine rolls during the papermaking process. Papermakers belts carry various names depending on their intended use. By way of example, papermakers belts include forming fabrics, wet press felts, and dryer felts and fabrics, to name just a few.
Forming fabrics, also known as fourdrinier wires, or forming media, are commonly configured from a length of woven fabric with its ends joined together in a seam to provide a endless belt. The fabric may also be constructed by employing an endless weave process, thereby eliminating the seam. Either fabric generally comprises a plurality of machine direction yarns and a plurality of cross machine direction yarns which have been woven together on a suitable loom.
Recently, in the papermaking field, it has been found that synthetic materials may be used, in whole or in part, to produce forming fabrics of superior quality. Today, almost all forming fabrics are made from polyesters such as Dacron or Trevina, acrylic fibers such as Orlon, Dynel and Acrylan, copolymers such as Saran, or polyamides such as Nylon. The warp and weft yarns of the forming fabric may be of the same or different constitute material and/or constructions, and may be in the form of a monofilament or multifilament yarn.
Among the problems prominent in conventionally woven forming fabrics are edge curling and control of fabric permeability without adversely affecting fabric strength and stability. Forming fabrics play the major role in producing a uniform paper product. In order to produce a uniform product, the forming fabric used must have consistent thickness, hole size, and draining properties, and there must be repeatability not only from fabric to fabric, but also within a fabric. Thus, a fabric structure and a construction method which allow repeated production of uniform fabrics are highly desirable. Some forming fabrics are chemically treated, after production, to improve properties such as sheet release, wear, and drainage, to name a few. The ability to manufacture a fabric with improved properties without the need for post-production chemical treatment would be advantageous.
A conventional dryer felt consists of an endless conveyor belt made from a two-or three-plane fabric wherein the various planes are defined by different groups of crossmachine direction yarns. These planes, plys or layers are united by a plurality of machine direction yarns. The yarns used to weave the most up-to-date dryer felts are made from synthetic monofilaments or synthetic multifilaments, formed of materials such as polyester or polyamide.
Because the synthetic materials are quite expensive, manufacturers are continually seeking to improve dryer felts, both in terms of cost and in terms of dryer felt performance and reliability. Along these lines, it is important in certain dryer felts to maintain low liquid permeability. One way of lowering the permeability is to weave more yarn to the inch, but this, of course, adds to the cost of the already expensive felts. Both the felt manufacturer and papermaker would benefit from a dryer felt in which low permeability could be provided at an economical cost.
Many of the prior art papermakers belts employ various types of seams which are joined together through the use of a pintle in order to facilitate insertion and removal of the papermakers belt from the papermaking machine. A very common type of seam is one employing metal clipper hooks arranged in an alternating relationship on either end of the papermakers belt to receive a pintle to join the ends of the belt together. In other papermakers belts, the ends of the belts are joined in an abutting relationship and held that way through the use of clipper hooks inserted in a webbing, the whole of which is attached to the underside of the belt. Such an arrangement has the disadvantage of frequently causing seam marking in the paper sheet being made. Another problem is that bouncing of the papermakers belt occurs when the seam either meets or leaves the contact of a machine roll. This bouncing causes further marking of the paper sheet and, in an extreme case, may cause paper sheet breaks and the rapid weakening of the fabric structure.
Woven papermakers fabrics frequently have variations in thickness over the fabric due to the weaving process. Because of the variation in thickness, wet felts must be produced as endless woven belts since potential differences in thickness of the fabric at the seam could cause bouncing at the press rolls, resulting in unacceptable paper characteristics. A fabric which can be produced with uniform thickness would have the advantage of being able to be spliced on the paper machine to produce an endless fabric without the threat of increased roll bounce. Downtime for fabric changes could be reduced by a fabric which is readily joined on the paper machine. Such a fabric would also reduce the roll bounce associated with irregularities over the length of the fabric, thus allowing production of higher quality paper.
Present woven fabrics often have different compressibility characteristics from one fabric to the next. Uniform compressibility is necessary for correct adjustment of the press roll nip so that there is correct moisture removal without damaging the paper. A fabric in which compressibility could be controlled and uniformly maintained during manufacture would have the advantage of requiring fewer adjustments to the paper machine when fabrics are changed, thus allowing increased production.
The ability to absorb and to give up moisture is an important characteristic for certain fabrics, particularly wet felts. This ability is influenced to a large extent by the capillary structure of the fabric. In the prior art, the choice of yarns and weave design gives some control over capillary structure, but it does not offer precise control. A fabric in which the capillary structure could be established, with a high degree of repeatability, during the manufacturing process, would have significant advantages over the prior art.
A further problem with present woven wet felts is that once a new wet felt is installed, a relatively long break-in time is required before the fabric becomes compacted and stabilized in its moisture removing configuration. There would be a significant advantage to a fabric which was precompacted during manufacture, as this would eliminate the break-in time thereby increasing the productivity of the paper machine.
There is thus a need for a papermakers belt which may function reliably in specific of the various environments encountered during the papermaking process while at the same time being produced inexpensively and efficiently. It is also desirable that the papermakers belt be endless in construction and easily repaired while in its position of intended use. The present invention is directed toward filling those needs.